1. Field of the Invention
This invention relates to steam generators for nuclear power plants and more particularly to steam generators having a feedwater distribution device disposed in the annular downcomer region of the steam generator.
2. Description of the Prior Art
A steam generator of conventional type includes a tube bundle positioned within a shell and encircled by a wrapper. An annular downcomer region is formed between the shell and the wrapper. In certain steam generators, feedwater is introduced into a feedwater inlet device positioned at an elevation above the tube bundle. Feedwater is introduced into the shell through an inlet nozzle, travels down the annular downcomer region, and up through the tube bundle where the feedwater is heated by primary fluid passing through the tubes of the tube bundle.
The introduction of cold feedwater into a steam region of the steam generator can create a water hammer event due to the possibility of the cold feedwater quenching saturated temperature steam present in the steam generator. The steam can instantaneously collapse, thereby producing shock waves and component loadings in the steam generator. Smaller scale bubble collapse phenomena can complicate controlling the water level in the steam generator. The introduction of cold feedwater directly into the tube bundle where steam may be present can present operational concerns. Also, contact of the cold feedwater with hotter pressure boundary components, such as the tubesheet or the shell, may produce undesirable thermal fatigue.
Currently, to limit the potential of water hammer or thermal shock to the steam generator, complex and expensive feedwater temperature and flow path monitoring and control equipment is used.
As disclosed in U.S. Pat. No. 4,357,908 issued Nov. 9, 1982 to Jean-Claude Yazidjian entitled "Steam Generator with Pre-heating," the cold feedwater can be mixed with the warmer recirculating water in a space within a double-walled wrapper. The inner wrapper, or secondary envelope, encircles the tube bundle and the outer wrapper, or skirt, providing a thermal barrier to the shell. However, this double-walled wrapper within the space between the secondary envelope and the outer wall of the vessel adds additional cost to the steam generator. Also, the feedwater inlet ring is positioned at an elevation above the tube bundle at an upper portion of the steam generator for mixing the feedwater and recirculating water at the top region of the annular downcomer.
Therefore, what is needed is a steam generator having a feedwater distribution means for uniformly mixing the cold feedwater, which can be introduced into the steam generator at any elevation within the downcomer region, with the warmer recirculating water in the downcomer region of the steam generator prior to entry of the mixture into the tube bundle for reducing the possibility of water hammer and thermal shock to the steam generator without the additional cost of unnecessary components.